A number of studies in the past decade have shown that breastfed babies are slightly “smarter” than those who are formula-fed, at least in terms of standard intelligence test scores. Which, of course, has led to a predictable, lengthy media spat as to why this might be so.

Is the “IQ gap” due to some unknown smartness-inducing quality of breast milk itself? Or some IQ-damaging substance in formula? Or might it be due to some other factor altogether–like the extended mother-baby contact that comes with nursing, or the fact that the breastfeeding mothers in the studies tended to have higher IQs themselves, and more money, than the formula-feeding moms?

A couple of recently published studies don’t settle that issue, but do show how incredibly difficult it is to make infant formula that comes anywhere close to providing all the benefits that breast milk and breastfeeding do.

The first study*, published by Ahmad Qawasmi and colleagues in Pediatrics on May 28, looks at two fatty acids, abbreviated DHA and AA, which are added to infant formula. DHA and AA are long-chain polyunsaturated fatty acids known to be critical to cognitive development both within the womb and in early childhood. Breast milk contains substantial amounts of DHA and AA; formula did not until supplementation began some years ago. Formula manufacturers quickly trumpeted the benefits of these new additives to infant brain growth. And though they didn’t say so directly in their ads, they hoped that DHA and AA would close the IQ gap between their products and breast milk.

It didn’t work out that way. According to Qawasmi’s team, which performed a meta-analysis of a dozen studies on the subject, the addition of DHA and AA to formula provided no measurable “intelligence boost” at one year of age compared with un-supplemented formula.

That’s not to say that DHA and AA shouldn’t be added to infant formula–anything that brings the composition of formula closer in line with breast milk is a good thing for the 70% of American babies who are drinking at least some of it by the age of six months.

It’s just that we’ll have to look elsewhere for the answer to the differences in cognitive development between breast and formula babies. Qawasmi suggests that other factors should be studied, such as the antimicrobial, anti-inflammatory and immune-boosting properties of breast milk.

What this and other studies do demonstrate, though, is that it’s pretty darn tough (and likely impossible) to commercially duplicate all the benefits of breastfeeding. After millions of years of tinkering, nature knows what it’s doing.

(*A look at the second study, about soy formula and cognitive development, is coming up…)